PDFs for Event Generators: Why? Stephen Mrenna CD/CMS Fermilab

Transcription

1 PDFs for Event Generators: Why? Stephen Mrenna CD/CMS Fermilab 1

2 Understanding Cross LHC: many pieces to the puzzle LO, NLO and NNLO calculations K-factors Benchmark cross sections and pdf correlations PDFs with uncertainties Underlying event and minimum bias Fragmentation/Hadronization Sudakov form factors Jet algorithms and jet reconstruction 2

3 Why NLO? Importance of NLO: PDF fitting and uncertainty Sensible output Precision cross section estimates Significantly reduces scale dependence and stabilizes shapes Limitations Inclusive enough observables Hard 3

4 K-factors: how important is NLO? Ignores shape changes K=NLO/LO 6M/6L1 6M/6M 4

5 Shape dependence of a K-factor PDF uncertainty Range is large Inclusive jet: Probes a wide range of x, Q Mixture of qq,gg, qg 5

6 The LHC: a very jetty place W+2j NLO tt~+j LO W+3j LO Top total inclusive NLO Has perturbation theory gone wrong? 6

7 Perturbation Theory 101: Feynman diagram approach Typical Hard Scale Q Typical pt/q > O(.1) s 4 O s 5, s ~.12 7

8 + additional soft/collinear gluons Soft/collinear if pt << Q s ~ 1 ln p T Q 8 s ln p T Q ln p T Q 1O N ln 2N, 2N 1 s

9 Perturbation Theory 102: Sudakov form factors t t=exp[ dt ' t0 t ' dz z S z,t ' 2 P z f x/ z,t ' f x,t ' ] Basis for resummation and parton showering Sums effects of soft and collinear gluon emission, but not large energy, wide angle gluon emission Initial state and final state logs summed separately FSR has no PDF reweighting FSR modeling tested extensively at LEP Gives the probability not to radiate a gluon greater than some energy 9

10 Event Generators use PDFs for: Setting kinematics at high Q Backwards evolution ISR Transverse evolution: P T, boson Underlying Event (semi-hard QCD,low x) Important for modeling: triggering, track occupancy, jet energy, isolation, etc. 10

11 Monte Carlo PDFs Which PDFs for parton shower Monte Carlos? standard to use LO PDFs, most commonly CTEQ5L/CTEQ6L, in Pythia, Herwig, Sherpa, ALPGEN/Madgraph+ Concerns: LO PDFs can create LHC cross sections/acceptances that differ in both shape and normalization from NLO due to influence of HERA data and lack of ln(1/x) and ln(1-x) terms in LO PDFs and evolution outside NLO error bands 11

12 CSS resummation parton shower? Parton shower? ME corr ~NLO PDF 12

13 Effect on Sudakov Not shown: UE variation Sudakov NLO PDF band very narrow Sudakov LO PDF Branching Prob Scale variation 13

14 PDF error treatment in MC NLO error PDFs are used in combination with the central LO PDF an error in PDF re-weighting due to nonmatching of Sudakov form factors f LO x,q f NLO,error x,q f NLO, central x,q Times a ratio of LO PDFs from ISR for each emission From ME in Alpgen, Pythia, etc PDF error estimate 14

15 Modified LO pdf s (LO*) but the low x behavior of LO PDFs are used in models of the underlying event (UE) at the Tevatron and its extrapolation to the LHC Also used for calculating low x cross sections at the LHC motivation for modified LO PDFs 15

16 16 Tunes with CTEQ6L

17 Reasonable behavior LO* PDFs should behave as LO as x->0; as close to NLO as possible as x->1 LO* PDFs should be universal and produce reasonable results out of the box It should be possible to produce error PDFs: similar Sudakov form factors similar UE so PDF re-weighting makes sense LO* PDFs should describe with a tune similar to CTEQ6L (for convenience) and extrapolate to a reasonable UE at the LHC 17

18 Where are the differences between LO and NLO partons? W + rapidity distribution at LHC U Quarks LO 6L1 K-factor=1.15 LO 6.1 NLO 6.1 W+ Rapidity 18 Missing ln(1-x) at LO LO 6L1 == (LO ME) (LO PDF) LO 6.1 == (LO ME) (NLO PDF) NLO 6.1 == (NLO ME) (NLO PDF)

19 Where are the differences:gluons? Gluon CTEQ5L and 6L steeper than 6.1 (or any NLO gluon pdf) at low x missing ln(1/x) terms in LO ME 19

20 MRSTLO* The MRST group has a modified LO pdf that tries to incorporate many of these points They relax the momentum sum rule (114%) and achieve a better agreement (than MRST LO pdf s) with some important LHC benchmark cross sections Available in LHAPDF 20

21 CTEQ variations INCLUDE IN LO* FIT (WEIGHTED) PSEUDO-DATA FOR CHARACTERISTIC LHC PROCESSES PRODUCED USING CTEQ6.6 NLO PDF S WITH NLO MATRIX ELEMENTS (USING MCFM) 21 Use of 2-loop or 1-loop α s Herwig preference for 2-loop Pythia preference for 1-loop Fixed momentum sum rule, or not re-arrange momentum within proton and/or add extra momentum extra momentum appreciated by some of pseudo-data sets but not others and may lose some useful correlations Fix pseudo-data normalizations to K-factors expected from higher order corrections, or let float Scale variation within reasonable range for finetuning of agreement with pseudo-data vector boson scale varies from 0.5 m B to 2.0 m B

22 22 Results: gluon distribution

23 23 Focus on small-x